Where is the Base?

ASCE 7-05 defined the base for seismic design as:

“The level at which the horizontal seismic ground motions are considered to be imparted to the structure”.

Most of the times this is misinterpreted or misunderstood that where the base is considered when determining the height of building for seismic calculations specially when the site is sloped or if the basement is open from one side.

Read the following article published in December 2009 issue of Structure magazine, this article will answer lots of questions.



Modeling Piles/Piers in FEM Software

When we use finite element software for analyzing deep foundations then it becomes important to model piles with accurate axial stiffness so that the load can distribute properly based on pile stiffnesses.

Some people suggests to calculate pile stiffness by AE/L formula. By doing this method the problem is that as you increase the length of the pile, the stiffness will reduce which is not accurate.

The best thing is to ask Geotech Engineer for the allowable loads and the allowable axial displacement on working loads. Most of the times this allowable displacement is ranged from ¼” to ½” which can be confirmed from geotech report.

Take your allowable load which will vary based on the depth of pile and divide it by the axial displacement, this will be the required pile stiffness.



k = Pile axial stiffness (kips/inch )

P = Allowable load (kips)

Δ = Allowable vertical displacement (inches)

SAP2000-Accidental Torsion in Response-Spectrum Case

NOTE: This discussion applies only to SAP2000. ETABS automatically includes the accidental eccentricity specified in the response-spectrum load case.

To consider accidental torsion in a response-spectrum case, eccentricity should be added as a separate static load case. The response-spectrum (SPEC) and eccentric (ECC) load cases should then be combined as SPEC + ECC and SPEC – ECC load combinations. Each level should have a rigid diaphragm, otherwise accidental eccentricity is not of significance.

Accidental torsion is applied through the following process:

  • Define a load case named ECC (or other) of Type = Quake, Self-weight multiplier = 0, and Auto = None.
  • Select any point on the diaphragm at each level.
  • Select Assign > Joint/Point Loads > Force > Moment Global ZZ, then specify the appropriate value for torsional moment.
  • Evaluate SPEC + ECC and SPEC – ECC load combinations. Note that the software already considers SPEC in both directions.

Accidental torsion may be considered without rigid diaphragms, though loading should be more distributed. Torsional moment is calculated at joint locations within a flexible diaphragm by first resolving the total lateral force in a given direction at each story level. This is done by summing the auto-seismic loads applied to joints within each story level. This force is then multiplied by the eccentricity to generate torsional moment (T). This torsion is then distributed to all joints at the story level in proportion to the joint mass as follows:

  • Torsion at a given joint at a story level = T * (Mass at the joint / Total mass of all joints at the story level)

It may be convenient to use an Excel spreadsheet to calculate torsion at joint locations. Using interactive database editing, data may then be entered into SAP2000

Source: https://wiki.csiberkeley.com/display/kb/Accidental+torsion+in+response-spectrum+case


I found this problem when I was importing HSS sections from AISC profile file, that the default material for HSS is not 46ksi and all the tube were imported with Fy=50ksi. It was tedious to go and change every HSS material one by one to Fy=46ksi.

Here is the procedure which should be followed in order to avoid this issue:

1. Click Section Properties > Frame Sections

2. Click “Import New Property” button

3. Click “Tube” button

4. Select file to import from computers and structures > SAP2000 folder. File name will be AISC13.pro for AISC 13th Edition in English units and or AISC13M.pro for metric units. (Also sizes per 14th edition is also included in new version)

5. The box will appear with sizes to import. Change the material to A500GrB46 or any material which is required.

Select sizes to import and click ok.

6. The material for all HSS tube will be A500GrB46.